AN OVERVIEW OF ORGANIC CHEMISTRY
Alkenes and Alkynes I.
Properties and Synthesis
7.1 Introduction
1. Alkenes are hydrocarbons whose molecules contain the C–C double bond.
1) olefin:
i) Ethylene was called olefiant gas (Latin: oleum, oil + facere, to make) because gaseous ethane (C2H4) reacts with chlorine to form C2H4Cl2, a liquid (oil).
[pic]
2. Alkynes are hydrocarbons whose molecules contain the C–C triple bond.
1) acetylenes:
7.1A Physical Properties of Alkenes and Alkynes
1. Alkenes and alkynes have physical properties similar to those of corresponding alkanes.
1) Alkenes and alkynes up to four carbons (except 2-butyne) are gases at room temperature.
2) Alkenes and alkynes dissolve in nonpolar solvents or in solvents of low polarity.
i) Alkenes and alkynes are only very slightly soluble in water (with alkynes being slightly more soluble than alkenes).
ii) Alkenes and alkynes have densities lower than that of water.
7.2 Nomenclature of Alkenes and Cycloalkenes
1. Determine the base name by selecting the longest chain that contains the double bond and change the ending of the name of the alkane of identical length from -ane to -ene.
2. Number the chain so as to include both carbon atoms of the double bond, and begin numbering at the end of the chain nearer the double bond. Designate the location of the double bond by using the number of the first atom of the double bond as a prefix:
3. Indicate the location of the substituent groups by numbering of the carbon atoms to which they are attached.
4. Number substituted cycloalkenes in the same way that gives the carbon atoms of the double bond the 1 and 2 positions and that also gives the substituent groups the lower numbers at the first point of difference.
5. Name compounds containing a double bond and an alcohol group as alkenols (or cycloalkenols) and give the alcohol carbon the lower number.
6. Two frequently encountered alkenyl groups are the vinyl group and allyl group.
7. If two identical groups are on the same side of the double bond, the compound can be designated cis; if they are on the opposite sides it can be designated trans.
7.2A The (E)-(Z) System for Designating Alkene
Diastereomers
1. Cis- and trans- designations the stereochemistry of alkene diasteroemers are unambiguous only when applied to disubstituted alkenes.
[pic]
2. The (E)-(Z) system:
[pic]
1) The group of higher priority on one carbon atom is compared with the group of higher priority on the other carbon atom:
i) (Z)-alkene: If the two groups of higher priority are on the same side of the double bond (German: zusammen, meaning together).
ii) (E)-alkene: If the two groups of higher priority are on opposite side of the double bond (German: entgegen, meaning opposite).
[pic]
[pic]
7.3 Relative Stabilities of Alkenes
7.3A Heats of Hydrogenation
1. The reaction of an alkene with hydrogen is an exothermic reaction; the enthalpy change involved is called the heat of hydrogenation.
1) Most alkenes have heat of hydrogenation near –120 kJ mol–1.
[pic] ΔH° ≈ – 120 kJ mol–1
2) Individual alkenes have heats of hydrogenation may differ from this value by more than 8 kJ mol–1.
3) The differences permit the measurement of the relative stabilities of alkene isomers when hydrogenation converts them to the same product.
[pic]
[pic]
[pic]
2. In each reaction:
1) The product (butane) is the same.
2) One of the reactants (hydrogen) is the same.
3) The different amount of heat evolved is related to different stabilities (different heat contents) of the individual butenes.
[pic]
Figure 7.1 An energy diagram for the three butene isomers. The order of stability is trans-2-butene > cis-2-butene > 1-butene.
4) 1-Butene evolves the greatest amount of heat when hydrogenated, and trans-2-butene evolves the least.
i) 1-Butene must have the greatest energy (enthalpy) and be the least stable isomer.
ii) trans-2-Butene must have the lowest energy (enthalpy) and be the most stable isomer.
3. Trend of stabilities: trans isomer > cis isomer
[pic]
[pic]
4. The greater enthalpy of cis isomers can be attributed to strain caused by the crowding of two alkyl groups on the same side of the double bond.
[pic]
Figure 7.2 cis- and trans-Alkene isomers. The less stable cis isomer has greater strain.
7.3B Relative Stabilities from Heats of Combustion
1. When hydrogenation os isomeric alkenes does not yield the same alkane, heats of combustion can be used to measure their relative stabilities.
1) 2-Methylpropene cannot be compared directly with other butene isomers.
[pic]
2) Isobutane and butane do not have the same enthalpy so a direct comparison of heats of hydrogenation is not possible.
2. 2-Methylpropene is the most stable of the four C4H8 isomers:
[pic]
[pic]
[pic]
[pic]
3. The stability of the butene isomers:
[pic]
7.3C Overall Relative Stabilities of Alkenes
1. The greater the number of attached alkyl groups (i.e., the more substituted the carbon atoms of the double bond), the greater is the alkene’s stability.
[pic]
7.4 Cycloalkenes
1. The rings of cycloalkenes containing five carbon atoms or fewer exist only in the cis form.
[pic] [pic] [pic] [pic]
Cyclopropene Cyclobutene Cyclopentene Cyclohexene
Figure 7.3 cis-Cycloalkanes.
2. There is evidence that trans-cyclohexene can be formed as a very reactive short-lived intermediate in some chemical reactions.
[pic]
Figure 7.4 Hypothetical trans-cyclohexene. This molecule is apparently too highly strained to exist at room temperature.
3. trans-Cycloheptene has been observed spectroscopically, but it is a substance with very short lifetime and has not been isolated.
4. trans-Cyclooctene has been isolated.
1) The ring of trans-cyclooctene is large enough to accommodate the geometry required by trans double bond and still be stable at room temperature.
2) trans-Cyclooctene is chiral and exists as a pair of enantiomers.
[pic] [pic]
cis-Cyclooctene trans-Cyclooctene
Figure 7.5 The cis- and trans forms of cyclooctene.
7.5 Synthesis of Alkenes via Elimination Reactions
1. Dehydrohalogenation of Alkyl Halides
[pic]
2. Dehydration of Alcohols
[pic]
3. Debromination of vic-Dibromides
[pic]
7.6 Dehydrohalogenation of Alkyl Halides
1. Synthesis of an alkene by dehydrohalogenation is almost always better achieved by an E2 reaction:
[pic]
2. A secondary or tertiary alkyl halide is used if possible in order to bring about an E2 reaction.
3. A high concentration of a strong, relatively nonpolarizable base, such alkoxide ion, is used to avoid E1 reaction.
4. A relatively polar solvent such as an alcohol is employed.
5. To favor elimination generally, a relatively high temperature is used.
6. Sodium ethoxide in ethanol and potassium tert-butoxide in tert-butyl alcohol are typical reagents.
7. Potassium hydroxide in ethanol is used sometimes:
OH– + C2H5OH [pic] H2O + C2H5O–
7.6A E2 Reactions: The Orientation of the Double Bond in the
Product Zaitsev’s Rule
1. For some dehydrohalogenation reactions, a single elimination product is possible:.
[pic]
[pic]
[pic]
2. Dehydrohalogenation of many alkyl halides yields more than one product:
[pic]
1) When a small base such as ethoxide ion or hydroxide ion is used, the major product of the reaction will be the more stable alkene.
[pic]
i) The more stable alkene has the more highly substituted double bond.
2. The transition state for the reaction:
[pic]
1) The transition state for the reaction leading to 2-methyl-2-butene has the developing character of a double bond in a trisubstituted alkene.
2) The transition state for the reaction leading to 2-methyl-1-butene has the developing character of a double bond in a disubstituted alkene.
3) Because the transition state leading to 2-methyl-2-butene resembles a more stable alkene, this transition state is more stable.
[pic]
Figure 7.6 Reaction (2) leading to the the more stable alkene occurs faster than reaction (1) leading to the less stable alkene; ΔG‡(2) is less than ΔG‡(1).
i) Because this transition state is more stable (occurs at lower free energy), the free energy of activation for this reaction is lower and 2-methyl-2-butene is formed faster.
4) These reactions are known to be under kinetic control.
3. Zaitsev rule: an elimination occurs to give the most stable, more highly substituted alkene
1) Russian chemist A. N. Zaitsev (1841-1910).
2) Zaitsev’s name is also transliterated as Zaitzev, Saytzeff, or Saytzev.
7.6B An Exception to Zaitsev’s Rule
1. A bulky base such as potassium tert-butoxide in tert-butyl alcohol favors the formation of the less substituted alkene in dehydrohalgenation reactions.
[pic]
1) The reason for leading to Hofmann’s product:
i) The steric bulk of the base.
ii) The association of the base with the solvent molecules make it even larger.
iii) tert-Butoxide removes one of the more exposed (1°) hydrogen atoms instead of the internal (2°) hydrogen atoms due to its greater crowding in the transition state.
7.6C The Stereochemistry of E2 Reactions: The Orientation
of Groups in the Transition State
1. Periplannar:
1) The requirement for coplanarity of the H–C–C–L unit arises from a need for proper overlap of orbitals in the developing π bond of the alkene that is being formed.
2) Anti periplannar conformation:
i) The anti periplannar transition state is staggered (and therefore of lower energy) and thus is the preferred one.
[pic]
3) Syn periplannar conformation:
i) The syn periplannar transition state is eclipsed and occurs only with rigid molecules that are unable to assume the anti arrangement.
2. Neomenthyl chloride and menthyl chloride:
[pic]
[pic]
1) The β-hydrogen and the leaving group on a cyclohexane ring can assume an anti periplannar conformation only when they are both axial:
[pic]
2) The more stable conformation of neomenthyl chloride:
i) The alkyl groups are both equatorial and the chlorine is axial.
ii) There also axial hydrogen atoms on both C1 and C3.
ii) The base can attack either of these hydrogen atoms and achieve an anti periplannar transition state for an E2 reaction.
ii) Products corresponding to each of these transition states (2-menthene and 1-menthene) are formed rapidly.
v) 1-Menthene (with the more highly substituted double bond) is the major product (Zaitsev’s rule).
A Mechanism for the Elimination Reaction of Neomenthyl Chloride
E2 Elimination Where There Are Two Axial Cyclohexane β-Hydrogens
[pic]
A Mechanism for the Elimination Reaction of Menthyl Chloride
E2 Elimination Where The Only Eligible Axial Cyclohexane β-Hydrogen is From a Less Stable Conformer
[pic]
3) The more stable conformation of menthyl chloride:
i) The alkyl groups and the chlorine are equatorial.
ii) For the chlorine to become axial, menthyl chloride has to assume a conformation in which the large isopropyl group and the methyl group are also axial.
ii) This conformation is of much higher energy, and the free energy of activation for the reaction is large because it includes the energy necessary for the conformational change.
ii) Menthyl chloride undergoes an E2 reaction very slowly, and the product is entirely 2-menthene (Hofmann product).
7.7 Dehydration of Alcohols
1. Dehydration of alcohols:
1) Heating most alcohols with a strong acid causes them to lose a molecule of water and form an alkene:
[pic]
2. The reaction is an elimination and is favored at higher temperatures.
1) The most commonly used acids in the laboratory are Brønsted acids ––– proton donors such as sulfuric acid and phosphoric acid.
2) Lewis acids such as alumina (Al2O3) are often used in industrial, fas phase dehydrations.
3. Characteristics of dehydration reactions:
1) The experimental conditions –– temperature and acid concentration –– that are required to bring about dehydration are closely related to the structure of the individual alcohol.
i) Primary alcohols are the most difficult to dehydrate:
[pic]
ii) Secondary alcohols usually dehydrate under milder conditions:
[pic]
iii) Tertiary alcohols are usually dehydrated under extremely mild conditions:
[pic]
iv) Relative ease of order of dehydration of alcohols:
[pic]
2) Some primary and secondary alcohols also undergo rearrangements of their carbon skeleton during dehydration.
i) Dehydration of 3,3-dimethyl-2-butanol:
[pic]
ii) The carbon skeleton of the reactant is
[pic]
7.7A Mechanism of Alcohol Dehydration: An E1 Reaction
1. The mechanism is an E1 reaction in which the substrate is a protonated alcohol (or an alkyloxonium ion).
Step 1 [pic]
1) In step 2, the leaving group is a molecule of water.
2) The carbon-oxygen bond breaks heterolytically.
3) It is a highly endergonic step and therefore is the slowest step.
Step 2 [pic]
Step 3 [pic]
7.7B Carbocation Stability and the Transition State
1. The order of stability of carbocations is 3° > 2° > 1° > methyl:
[pic]
A Mechanism for the Reaction
Acid-Catalyzed Dehydration of Secondary or Tertiary Alcohols: An E1 Reaction
Step 1
[pic]
Step 2
[pic]
Step 3
[pic]
2. The order of free energy of activation for dehydration of alcohols is 3° > 2° > 1° > methyl:
[pic]
Figure 7.7 Free-energy diagrams for the formation of carbocations from protonated tertiary, secondary, and primary alcohols. The relative free energies of activation are tertiary < secondary « primary.
3. Hammond-Leffler postulate:
1) There is a strong resemblance between the transition state and the cation product.
2) The transition state that leads to the 3° carbocation is lowest in free energy because it resembles the most stable product.
3) The transition state that leads to the 1° carbocation is highest in free energy because it resembles the least stable product.
4. Delocalization of the charge stabilizes the transition state and the carbocation.
[pic]
1) The carbon begins to develop a partial positive charge because it is losing the electrons that bonded it to the oxygen atom.
2) This developing positive charge is most effectively delocalized in the transition state leading to a 3° carbocation because of the presence of three electron-releasing alkyl groups.
[pic]
3) Because this developing positive charge is least effectively delocalized in the transition state leading to a 1° carbocation, the dehydration of a 1° alcohol proceeds through a different mechanism ––– an E2 mechanism.
7.7C A Mechanism for Dehydration of Primary Alcohols:
An E2 Reaction
A Mechanism for the Reaction
Dehydration of a Primary Alcohol: An E2 Reaction
[pic]
[pic]
7.8 Carbocation Stability and the Occurrence of
Molecular Rearrangements
7.8A Rearrangements During Dehydration of Secondary
Alcohols
[pic]
Step 1 [pic]
Step 2 [pic]
1. The less stable, 2° carbocation rearranges to a more stable 3° carbocation.
Step 3 [pic]
2. The methyl group migrates with its pair of electrons, as a methyl anion, –:CH3 (a methanide ion).
3. 1,2-Shift:
4. In the transition state the shifting methyl is partially bonded to both carbon atoms by the pair of electrons with which it migrates. It never leaves the carbon skeleton.
5. There two ways to remove a proton from the carbocation:
1) Path (b) leads to the highly stable tetrasubstituted alkene, and this is the path followed by most of the carbocations.
2) Path (a) leads to a less stable, disubstituted alkene and produces the minor product of the reaction.
3) The formation of the more stable alkene is the general rule (Zaitsev’s rule) in the acid-catalyzed dehydration reactions of alcohols.
Step 4 [pic]
6. Rearrangements occur almost invariably when the migration of an alkanide ion or hydride ion can lead to a more stable carbocation.
[pic]
[pic]
[pic]
7.8B Rearrangements After Dehydration of a Primary
Alcohol
1. The alkene that is formed initially from a 1° alcohol arises by an E2 mechanism.
1) An alkene can accept a proton to generate a carbocation in a process that is essentially the reverse of the deprotonation step in the E1 mechanism for dehydration of an alcohol.
2) When a terminal alkene protonates by using its π electrons to bond a proton at the terminal carbon, a carbocation forms at the second carbon of the chain (The carbocation could also form directly from the 1° alcohol by a hydride shift from its β-carbon to the terminal carbon as the protonated hydroxyl group departs).
3) Various processes can occur from this carbocation:
i) A different β-hydrogen may be removed, leading to a more stable alkene than the initially formed terminal alkene.
ii) A hydride or alkanide rearrangement may occur leading to a more stable carbocation, after which elimination may be completed.
iii) A nucleophile may attack any of these carbocations to form a substitution product.
v) Under the high-temperature conditions for alcohol dehydration the principal products will be alkenes rather than substitution products.
A Mechanism for the Reaction
Formation of a Rearranged Alkene During Dehydration of a Primary Alcohol
[pic]
[pic]
[pic]
7.9 Alkenes by Debromination of Vicinal Dibromides
1. Vicinal (or vic) and geminal (or gem) dihalides:
[pic]
1) vic-Dibromides undergo debromination:
[pic]
[pic]
A Mechanism for the Reaction
Mechanism:
Step 1
[pic]
Step 2
[pic]
1. Debromination by zinc takes place on the surface of the metal and the mechanism is uncertain.
1) Other electropositive metals (e.g., Na, Ca, and Mg) also cause debromination of vic-dibromide.
2. vic-Debromination are usually prepared by the addition of bromine to an alkene.
3. Bromination followed by debromination is useful in the purification of alkenes and in “protecting” the double bond.
7.10 Synthesis of Alkynes by Elimination Reactions
1. Alkynes can be synthesized from alkenes.
[pic]
1) The vic-dibromide is dehydrohalogenated through its reaction with a strong base.
2) The dehydrohalogenation occurs in two steps. Depending on conditions, these two dehydrohalogenations may be carried out as separate reactions, or they may be carried out consecutively in a single mixture.
i) The strong base, NaNH2, is capable of effecting both dehydrohalogenations in a single reaction mixture.
ii) At least two molar equivalents of NaNH2 per mole of the dihalide must be used, and if the product is a terminal alkyne, three molar equivalents must be used because the terminal alkyne is deprotonated by NaNH2 as it is formed in the mixture.
iii) Dehydrohalogenations with NaNH2 are usually carried out in liquid ammonia or in an inert medium such as mineral oil.
A Mechanism for the Reaction
Dehydrohalogenation of vic-Dibromides to form Alkynes
Reaction:
[pic]
Mechanism:
Step 1
[pic]
Step 2
[pic]
2. Examples:
[pic]
3. Ketones can be converted to gem-dichloride through their reaction with phosphorus pentachloride which can be used to synthesize alkynes.
[pic]
7.11 The Acidity of Terminal Alkynes
1. The hydrogen atoms of ethyne are considerably more acidic than those of ethane or ethane:
[pic]
1) The order of basicities of anions is opposite that of the relative acidities of the hydrocarbons.
Relative Basicity of ethanide, ethenide, and ethynide ions:
CH3CH2:– > CH2=CH:– > HC≡C:–
Relative Acidity of hydrogen compounds of the first-row elements of the periodic table:
H–OH > H–OR > H–C≡CR > H–NH2 > H–CH=CH2 > H–CH2CH3
Relative Basicity of hydrogen compounds of the first-row elements of the periodic table:
–:OH < –:OR < –:C≡CR < –:NH2 < –:CH=CH2 < –:CH2CH3
2) In solution, terminal alkynes are more acidic than ammonia, however, they are less acidic than alcohols and are less acidic than water.
3) In the gas phase, the hydroxide ion is a stronger base than the acetylide ion.
i) In solution, smaller ions (e.g., hydroxide ions) are more effectively solvated than larger ones (e.g., ethynide ions) and thus they are more stable and therefore less basic.
ii) In the gas phase, large ions are stabilized by polarization of their bonding electrons, and the bigger a group is the more polarizable it will be and consequently larger ions are less basic
7.12 Replacement of the Acetyleneic Hydrogen Atom of
Terminal Alkynes
1. Sodium alkynides can be prepared by treating terminal alkynes with NaNH2 in liquid ammonia.
H–C≡C–H + NaNH2 [pic] H–C≡C:– Na+ + NH3
CH3C≡C–H + NaNH2 [pic] CH3C≡C:– Na+ + NH3
1) The amide ion (ammonia, pKa = 38) is able to completely remove the acetylenic protons of terminal alkynes (pKa = 25).
2. Sodium alkynides are useful intermediates for the synthesis of other alkynes.
[pic]
Sodium alkynide 1° Alkyl halide Mono- or disubstituted acetylene
[pic]
3-Hexyne (75%)
3. An SN2 reaction:
[pic]
4. This synthesis fails when secondary or tertiary halides are used because the alkynide ion acts as a base rather than as a nucleophile, and the major results is an E2 elimination.
[pic]
7.13 Hydrogenation of Alkenes
1. Catalytic hydrogenation (an addition reaction):
1) One atom of hydrogen adds to each carbon of the double bond.
2) Without a catalyst the reaction does not take place at an appreciable rate.
CH2=CH2 + H2 [pic] CH3–CH3
CH3CH=CH2 + H2 [pic] CH3CH2–CH3
2. Saturated compounds:
3. Unsaturated compounds:
4. The process of adding hydrogen to an alkene is a reduction.
7.14 Hydrogenation: The Function of the Catalyst
1. Hydrogenation of an alkene is an exothermic reaction (ΔΗ° ≈ –120 kJ mol–1).
R–CH=CH–R + H2 [pic] R–CH2–CH2–R + heat
1) Hydrogenation reactions usually have high free energies of activation.
2) The reaction of an alkene with molecular hydrogen does not take place at room temperature in the absence of a catalyst, but it often does take place at room temperature when a metal catalyst is added.
[pic]
Figure 7.8 Free-energy diagram for the hydrogenation of an alkene in the presenceof a catalyst and the hypothetical reaction in the absence of a catalyst. The free energy of activation [ΔG‡(1)] is very much larger than the largest free energy of activation for the catalyzed reaction [ΔG‡(2)].
2. The most commonly used catalysts for hydrogenation (finely divided platinum, nickel, palladium, rhodium, and ruthenium) apparently serve to adsorb hydrogen molecules on their surface.
1) Unpaired electrons on the surface of the metal pair with the electrons of hydrogen and bind the hydrogen to the surface.
2) The collision of an alkene with the surface bearing adsorbed hydrogen causes adsorption of the alkene.
3) A stepwise transfer of hydrogen atoms take place, and this produces an alkane before the organic molecule leaves the catalyst surface.
4) Both hydrogen atoms usually add form the same side of the molecule (syn addition).
[pic]
Figure 7.9 The mechanism for the hydrogenation of an alkene as catalyzed by finely divided platinum metal: (a) hydrogen adsorption; (b) adsorption of the alkene; (c) and (d), stepwise transfer of both hydrogen atoms to the same face of the alkene (syn addition).
[pic]
7.14A Syn and Anti Additions
1. Syn addition:
[pic]
2. Anti addition:
[pic]
7.15 Hydrogenation of Alkynes
1. Depending on the conditions and the catalyst employed, one or two molar equivalents of hydrogen will add to a carbon–carbon triple bond.
1) A platinum catalyst catalyzes the reaction of an alkyne with two molar equivalents of hydrogen to give an alkane.
CH3C≡CCH3 [pic] [CH3CH=CHCH3] [pic] CH3CH2CH2CH3
7.15A Syn Addition of Hydrogen: Synthesis of Cis-Alkenes
1. A catalyst that permits hydrogenation of an alkyne to an alkene is the nickel boride compound called P-2 catalyst.
[pic]
1) Hydrogenation of alkynes in the presence of P-2 catalyst causes syn addition of hydrogen to take place, and the alkene that is formed from an alkyne with an internal triple bond has the (Z) or cis configuration.
2) The reaction take place on the surface of the catalyst accounting for the syn addition.
[pic]
2. Lindlar’s catalyst: metallic palladium deposited on calcium carbonate and is poisoned with lead acetate and quinoline.
[pic]
7.15B Anti Addition of Hydrogen: Synthesis of Trans-Alkenes
1. An anti addition of hydrogen atoms to the triple bond occurs when alkynes are reduced with lithium or sodium metal in ammonia or ethylamine at low temperatures.
1) This reaction, called a dissolving metal reduction, produces an (E)- or trans-alkene.
[pic]
A Mechanism for the Reduction Reaction
The Dissolving Metal Reduction of an Alkyne
[pic]
[pic]
7.16 Molecular Formulas of Hydrocarbons:
The Index of Hydrogen Deficiency
1. 1-Hexene and cyclohexane have the same molecular formula (C6H12):
CH2=CHCH2CH2CH2CH3 [pic]
1-Hexene Cyclohexane
1) Cyclohexane and 1-hexene are constitutional isomers.
2. Alkynes and alkenes with two double bonds (alkadienes) have the general formula CnH2n–2.
1) Hydrocarbons with one triple bond and one double bond (alkenynes) and alkenes with three double bonds have the general formula CnH2n–4.
CH2=CH–CH=CH2 CH2=CH–CH=CH–CH=CH2
1,3-Butadiene (C4H6) 1,3,5-Hexatriene (C6H8)
3. Index of Hydrogen Deficiency (degree of unsaturation, the number of double-bond equivalence):
1) It is an important information about its structure for an unknown compound.
2) The index of hydrogen deficiency is defined as the number of pair of hydrogen atoms that must be subtracted from the molecular formula of the corresponding alkane to give the molecular formula of the compound under consideration.
3) The index of hydrogen deficiency of 1-hexene and cyclohexane:
C6H14 = formula of corresponding alkane (hexane)
C6H12 = formula of compound (1-hexene and cyclohexane)
H2 = difference = 1 pair of hydrogen atoms
Index of hydrogen deficiency = 1
4. Determination of the number of rings:
1) Each double bond consumes one molar equivalent of hydrogen; each triple bond consumes two.
2) Rings are not affected by hydrogenation at room temperature.
CH2=CH(CH2)3CH3 + H2 [pic] CH3(CH2)4CH3
[pic] + H2 [pic] No reaction
[pic] + H2 [pic] [pic]
Cyclohexene
CH2=CHCH=CHCH2CH3 + 2 H2 [pic] CH3(CH2)4CH3
1,3-Hexadiene
4. Calculating the index of Hydrogen Deficiency (IHD):
1) For compounds containing halogen atoms: simply count the halogen atoms as hydrogen atoms.
C4H6Cl2 = C4H8 ( IHD = 1
2) For compounds containing oxygen atoms: ignore the oxygen atoms and calculate the IHD from the remainder of the formula.
C4H8O = C4H8 ( IHD = 1
CH2=CHCH2CH2OH CH3CH2=CHCH2OH [pic]
[pic] [pic] [pic] and so on
3) For compounds containing nitrogen atoms: subtract one hydrogen for each nitrogen atom, and then ignore the nitrogen atoms.
C4H9N = C4H8 ( IHD = 1
CH2=CHCH2CH2NH2 CH3CH2=CHCH2NH2 [pic]
[pic] [pic] [pic] and so on
Summary of Methods for the Preparation
of Alkenes and Alkynes
1. Dehydrohalogenation of alkyl halides (Section 7.6)
General Reaction
[pic]
Specific Examples
[pic]
[pic]
2. Dehydration of alcohols (Section 7.7 and 7.8)
General Reaction
[pic]
Specific Examples
CH3CH2OH [pic] CH2=CH2 + H2O
[pic] + H2O
Summary of Methods for the Preparation
of Alkenes and Alkynes
3. Debromination of vic-dibromides (Section 7.9)
General Reaction
[pic]
4. Hydrogenation of alkynes (Section 7.15)
General Reaction
[pic]
5. Dehydrohalogenation of vic-dibromides (Section 7.15)
General Reaction
[pic]
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